The present application claims priority under 35 U.S.C. xc2xa7119 from Korean patent application No. 2000-1550, xe2x80x9cIsolation Method for Single Crystalline Silicon in Micromachining Using Deep Trench Insulation Layers,xe2x80x9d filed with the Korean Industrial Property Office on Jan. 13, 2000.
1. Field of the Invention
The present invention relates generally to microelectromechanical systems and more specifically to such systems that include an electrical isolation structure and to methods for electrically isolating a part of a single crystalline silicon microstructure using a deep trench insulation layer in a microelectromechanical system.
2. Description of the Related Art
Microelectromechanical systems include component structures with typical minimum dimensions on the order of a micron where the component structures can have elaborate shapes and perform a variety of complex functions. The component structures of microelectromechnical systems are formed on a semiconductor or glass substrate. Microelectromechanical systems include devices such as accelerometers that sense the acceleration of a moving object, gyroscopes that sense the angular rate of a rotating object and mirror arrays that deflect light in fiber optic communication and display applications. Micromachining techniques are used to fabricate the very small structures that are integrated with electrical parts on the semiconductor or glass substrate. The techniques used to fabricate these microelectromechanical systems are largely based on semiconductor device fabricating technology, including photolithography, thin film deposition, etching, impurity doping by diffusion and ion implantation, electroplating and wafer bonding.
Microelectromechanical systems often include moving parts that are suspended from or tethered to an underlying substrate and that can move independently of the underlying substrate. Microelectromechanical systems also include electrodes that are electrically isolated to allow the electrodes, for example, to measure electrical signals flowing in the moving parts of the system. Other types of electrodes are used to apply electrical signals to the moving parts of the system; this application also requires that the electrodes be electrically isolated. Electrodes have to be electrically isolated from one another, and also from the substrate on which the electrodes and the tethered moving parts are fabricated. Many methods for electrically isolating a part of a microelectromechanical system from other parts of the system have been studied.
FIG. 1 shows process steps in the conventional isolation process known as the single crystalline reactive etching and metallization (hereinafter, referred to as xe2x80x9cSCREAMxe2x80x9d) process. The SCREAM isolation method fabricates a structure by the SCREAM micromachining technique in the manner discussed in U.S. Pat. No. 5,563,343; U.S. Pat. No. 5,198,390; and K. A. Shaw, Z. L. Zhang, and N. C. MacDonald, xe2x80x9cSCREAM I: A Single Mask, Single-Crystal Silicon, Reactive Ion Etching Process for Microelectromechanical Structures,xe2x80x9d Sensors and Actuators A, Vol. 40, pp. 63, 1994. Plasma enhanced chemical vapor deposition (hereinafter, xe2x80x9cPECVDxe2x80x9d) covers all surfaces of a micromachined structure with an oxide film. Selective deposition of metal film on the structure forms electrodes and electrically conducting paths on top of the PECVD. oxide film. In this SCREAM process, electrical isolation of the electrodes is achieved by depositing the metal film only on the top and the side surfaces of microelectromechanical structures.
The SCREAM isolation method has the advantage of being relatively simple in not requiring separate photolithography and etching steps once the structure is fabricated using the SCREAM micromachining technique. On the other hand, the coverage achieved in the deposition of the metal film is generally poor and hence the SCREAM isolation method typically cannot be applied to tall structures having a high aspect ratio. It should be noted that, if a metal or other material is deposited that has good step coverage, such as metal films deposited by low pressure chemical vapor deposition (hereinafter, referred to as xe2x80x9cLPCVDxe2x80x9d), all electrodes and microelectromechanical parts are electrically connected, and hence, electrical isolation is not achieved.
FIG. 2 shows the silicon on oxide insulator (hereinafter, xe2x80x9cSOIxe2x80x9d) wafer method, used in forming the microelectromechanical systems described in the following references: B. Diem, et al., xe2x80x9cSOI(SIMOX) as a Substrate for Surface Micromachining of Single Crystalline Silicon Sensors and Actuators,xe2x80x9d Tech. Dig. 7th Int. Conf. Solid-State Sensors and Actuators (Transducers ""93), Yokohama, Japan, 1993, pp. 233-236; and C. Marxer, et al., xe2x80x9cVertical Mirrors Fabricated by Deep Reactive Ion Etching for Fiber-Optic Switching Applications,xe2x80x9d IEEE/ASME Journal of Microelectromechanical Systems, Vol. 6, No. 3, pp. September 1997. In the SOI wafer method, the portion of the wafer on top of the buried oxide layer (hereinafter, the xe2x80x9cdevice layerxe2x80x9d) is highly doped, conducting silicon. Since all structures and electrodes are fabricated in the device layer and are defined by etching the device layer to the buried oxide layer, electrical isolation of the resulting electrodes is achieved automatically. On the other hand, SOI wafers are generally expensive and the residual stress created by the buried oxide layer can warp and change the shape of microelectromechanical structures. In addition, the micromachined portions of the device layer silicon near the oxide interface can have roughened features (known as the xe2x80x9cfootingxe2x80x9d effect) when the structures and electrodes are formed in a deep plasma etching process. Another disadvantage of the SOI process is that the as-manufactured wafer has an established thickness of the oxide film and the device layer and these thicknesses cannot be modified once a wafer is manufactured.
FIG. 3 shows a scanning electron microscope (SEM) photograph of a micromachined comb-drive structure fabricated from single crystal silicon. The electrodes of the comb-drive structure of the illustrated structure are isolated using the junction isolation method. The junction isolation method is described, for example, in S. Lee, S. Park and D. Cho, xe2x80x9cThe Surface/Bulk Micromachining (SBM) Process: A New Method for Fabricating Released Microelectromechanical Systems in Single Crystal Silicon,xe2x80x9d IEEE/ASME J. Microelectromechanical Systems, Vol. 8, No. 4, December 1999. The junction isolation method forms a junction diode on a lightly doped N-type or P-type wafer. Applying a reverse biased voltage to the junction diode isolates the junction electrode from the substrate. Referring to FIG. 3, the silicon substrate 10 is lightly doped P-type and the lighter parts 12, including the comb-drive structure, are highly doped with phosphorus, such that a PN junction between the silicon substrate 10 (P-type) and the electrode 12 (N-type) is formed. In this case, if a reverse bias voltage is applied to the PN junction, the electrodes 12 are electrically isolated from the silicon substrate 10. This method has the advantage that the isolation steps are done before the micromechanical structure is fabricated, so that the structure can be fabricated in a relatively easy manner. On the other hand, the method has disadvantage that the depth of the PN junction often cannot be made sufficiently deep, so that this process usually is not readily applied to a tall structure having a high aspect ratio.
FIG. 4 is a structure formed by yet another conventional isolation method, the trench oxide isolation method, described in the following references: U.S. Pat. No. 5,930,595; U. Sridhar et al., xe2x80x9cTrench Oxide Isolated Single Crystal Silicon Micromachined Accelerometer,xe2x80x9d IEEE IEDM, San Francisco Calif., Dec. 6-9, 1998. pp. 475-478; and S. Lee, S. Park, D. Cho and Y. Oh xe2x80x9cSurface/Bulk Micromachining (SBM) Process and Deep Trench Oxide Isolation Method for MEMSxe2x80x9d, IEEE IEDM, Washington, D.C., Dec. 5-8, 1999. pp. 701-704. This trench isolation method includes forming U-shaped trenches 14 on a silicon substrate 16, forming thermal oxide layers 18 and depositing oxide layers 20 on all sides of the structure where the trenches are formed. The oxide films 18, 20 filling the trenches attach the electrode structures 22, 24 to the silicon substrate 16 through the respective sidewalls so that the oxide films support the electrodes and tethered structures. The oxide films electrically isolate the electrodes from each other and from the substrate.
This method has the advantage that the method can be applied to a tall structure having a high aspect ratio. On the other hand, separate photolithography and etching steps undesirably are required to form a metal layer on the electrode, which is required for wire bonding the electrode to a package. Two different release processes are required: one to separate the electrode, component from the substrate and a second to separate the structure part from the substrate. The trenches between the sidewalls of the electrode and the sidewalls of the substrate generally cannot be made arbitrarily large, as would be desired to achieve a small parasitic capacitance, without sacrificing the structural rigidity of the trench filled oxide layers that support the structure and electrodes.
Additionally, the insulation layers the conventional trench isolation method deposits on the sides of the electrode are used to support the structure and electrodes. Therefore, the insulation layers must be deposited between the electrode and the substrate, which in turn, results in severe limitations on electrode shapes that can be fabricated. In particular, it is difficult to fabricate an electrode in an xe2x80x9cislandxe2x80x9d shape. Therefore, it can be appreciated that this method is difficult to use in microelectromechanical systems having a complicated electrode arrangement. Those skilled in the art can appreciate the need for a simpler isolation method.
An aspect of the present invention is an isolation method for microelectromechanical systems. The method includes etching a trench into a substrate, the trench having a depth at least as large as a sum of a thickness of an electrode to be formed and a separation distance between the electrode to be formed and an underlying surface of the substrate, the trench formed in an interior portion of the electrode to be formed. An insulation layer is formed within the trench. The electrode is patterned by etching around a periphery of the electrode to a depth greater than a thickness of the electrode. The substrate is laterally etched below the electrode, to at least partially separate the electrode from the underlying substrate, leaving the insulation layer in at least part of the trench so that the insulation layer anchors the electrode to the substrate and provides support for the electrode.
Another aspect of the present invention is an isolation method for microelectromechanical systems made of silicon, using deep trench insulation layers. The isolation method includes forming trenches to have a depth larger than the sum of the thickness of electrodes to be formed and a vertical separation between the electrodes to be formed and the underlying substrate. The trenches are formed in middle portions of the electrodes to be formed. The trenches are filled with insulation and the electrodes and etch holes are patterned. The electrodes are separated from the underlying substrate by etching the substrate laterally, below the electrodes and through the etch holes, so that the insulation in the trenches provides support for the electrodes spaced from the underlying substrate.
Yet another aspect of the present invention provides a microelectromechanical system including a substrate having two electrodes formed above a recessed surface of the substrate, each of the electrodes have a lower surface facing and spaced from the recessed surface. Trenches extend at least partially through corresponding ones of the electrodes and into the substrate at positions corresponding to the trenches through the electrodes. The trenches are formed in an interior portion of the electrodes. Insulation fills each of the trenches within the electrodes and the corresponding trenches in the substrate, the insulation extending between the electrodes and the substrate to support the electrodes and anchor the electrodes to the substrate. A peripheral separation between the electrodes is at least partially filled with air.